A common problem associated with intensive care hospitalization is the development of gastric ulcers in patients who are substantially moribund. This condition effects thousands of patients each day, and it must be aggressively treated to prevent the development of ulcers in the gastric mucosa, that can lead to hemorrhage and death if not prevented. For many years, neutralizing agents consisting of hydrated magnesium or aluminum hydroxide were routinely instilled in the stomach. This method, although partially effective, was complicated and messy. Further, the neutralizer had to be instilled frequently, and the presence of the excess fluid in the stomach placed the patient at increased risk to aspiration of gastric contents into the lungs.
The discovery of a class of pharmaceutical agents known as H.sub.2 antagonists has had a major impact on the prevention of stress related gastric mucosal damage (SRGMD) in critically ill patients. Proper use of these agents has been shown to effectively prevent the formation of stress related ulcers in a significant percentage of critically ill patients. However, acid production in the stomach is known to vary over time in each individual, and predicting when acid production is high, and how much antagonist an individual needs, is difficult. These agents, if given in a high dosage, can cause complete cessation of gastric acid production, in effect permitting the stomach to become a neutral environment which then can support the growth of pathogens. In the critically ill patient, aspiration of gastric contents into the lung occurs frequently, and several recent reports have shown an increase in pneumonia associated with the use of H.sub.2 antagonists. Pneumonia in the critically ill patient is a life threatening condition. It is, therefore, imperative to monitor and control gastric fluid pH within therapeutically defined limits, to prevent a highly acidic condition, without promoting the growth of pathogens.
The intermittent monitoring of gastric pH is common in the practice of critical care medicine. This monitoring technique is focused on patients who are intubated with a nasogastric tube and who are critically ill. If their gastric pH is not controlled, up to 75% of these patients can develop significant SRGMD. The current method of monitoring pH consists of manually withdrawing a sample of gastric contents via the nasogastric tube and exposing the sample to litmus paper. Litmus paper develops color in relation to the pH of the applied sample, and the pH is estimated by comparing the color developed to the color of a calibration scale. Not only is this process time consuming and of limited accuracy, it also exposes the care giver to potentially hazardous material.
Electrochemical methods for determining pH have also been generally available for many years. These techniques involve the immersion of a sensor electrode, usually a pH sensitive glass, into the sample to be measured, along with a reference electrode to complete a measuring circuit. The electrode pair is calibrated before and after use by cleaning and exposing to solutions of known pH. Some of these electrochemical methods are known to exist for measuring gastric pH, involving the insertion of a miniature sensor directly into the stomach. Because of the risk and difficulty in placing such devices into the patient's gastrointestinal tract, the sensors must be of high quality and expensive construction, in order to minimize the effects of electrode drift over long periods of time.
Electrode drift is the time dependent change in the relationship between actual pH and the indicated pH. This is particularly important where gastric pH must be monitored over a 24 to 72 hour period. Even a relatively low drift rate for an in vitro sensor will be unacceptable, because of the length of the period of monitoring during which recalibration is not possible. At least one sensor designated to minimize this problem has a single crystal of antimony, prepared in a specific orientation on the tip of an electrode lead which can be inserted into a catheter. Miniature glass electrodes have also been utilized for in vitro sensing, but they are known to be fragile and subject to drift problems. Chemically sensitive field effect transistors and fiber optic photochemical sensors have also been developed for in vitro sensing.
In addition to the difficulty of having to frequently remove such endoscopic sensors for recalibration, their very design as miniature devices makes them better suited for determining pH at a local site, rather than as sensors for finding an average pH for the entire stomach. The production of acid is known to vary from site to site along the gastric mucosa, and as a consequence, the positioning of the in vivo sensor is critical to obtaining representative readings. Unfortunately, it is not possible to easily control the position of such catheters over several days of observation.
Although these teachings suggest great improvements can be obtained in measurement of gastric pH, they do not address how such complicated devices can be manufactured at a low cost and applied in a manner that gives a drift free, accurate, and representative average value of gastric pH that is appropriate for use in controlling gastric pH.